1. Field of the Invention
The present invention relates to a crystal oscillating circuit, and more particularly to a fast wake-up crystal oscillating circuit capable of assuring successful oscillation.
2. Description of Related Art
FIG. 1 shows a conventional crystal oscillating circuit comprising an inverting amplifier 11, a feedback resistor 12, a crystal oscillator 13, a pair of capacitors 14 and a Schmitt trigger 16. The inverting amplifier 11 is employed for signal amplification to provide a gain for the crystal oscillating circuit. The Schmitt trigger 16 is triggered by an amplified signal generated by the inverting amplifier 11 to output a clock signal CLK. The feedback resistor 12 supplies a DC operation bias to the inverting amplifier 11. The crystal oscillator 13 generates an oscillation signal having a resonance frequency. The pair of capacitors 14 causes the crystal oscillating circuit to have a parallel resonance very close to a series resonance while the oscillation frequency of the crystal oscillating circuit is between the parallel resonance and the series resonance.
For the aforesaid crystal oscillating circuit, the signal bandwidth and amplification gain vary with operation voltage, processing parameters and load capacitor pair. In practical applications, these variations may cause difficulty or even failure for the crystal oscillating circuit to wake up.
To solve the above-mentioned problems, the prior art provided an improved crystal oscillating circuit as shown in FIG. 2, which utilizes a number of parallel inverting amplifiers 11 to achieve the object to fast wake up the crystal oscillating circuit. FIG. 3 shows the switching operation of the inverting amplifiers 11 of FIG. 2. First, all the inverting amplifiers 11 are turned on when a power supply is applied (step S301) (known as “strong mode”). In the strong mode, the amplification gain of the amplifiers 11 is very high so that the object to fast wake up the crystal oscillating circuit can be achieved. However, the current consumed by the amplifiers at that time is also high. To save the power consumption, the clock CLK so generated is applied to an N-bit counter 17 (step S302). After 2N clocks, the counter 17 stops counting, remaining only a basic inverting amplifier 11 for keeping the crystal oscillating circuit to oscillate (known as “weak mode”), while the other inverting amplifiers 11 are turned off (step S303).
The aforesaid improved crystal oscillating circuit enables fast wake-up by connecting plural inverting amplifiers in parallel. However, after wake-up, only one basic inverting amplifier 11 is retained, and thus the wake-up crystal oscillating circuit also encounters the same problem as the conventional crystal oscillating circuit in that the crystal oscillating circuit may fail to wake up due to operation voltage, processing parameter and load capacitor variations. Therefore, it is desirable to provide an improved crystal oscillating circuit to mitigate and/or obviate the aforementioned problems.